The present invention relates to blood pressure measuring systems; and more particularly it relates to a system for measuring blood pressure and for generating a visual display representative of instantaneous blood pressure.
A number of ways have been suggested for measuring blood pressure, but there are two systems which currently have particularly widespread commercial use. One such system is the conventional pressure cuff which is used by general physicians and other technical personnel for obtaining a rough measure of systolic and diastolic pressures. In general, the systolic pressure is the maximum pressure occurring during ventricle contraction, and the diastolic pressure is a minimum pressure which occurs when the ventricle dilates. Because of the limitations on the instrument and the fact that the cuff is placed over an arm and a reading taken by ear, the resulting measurements are only estimates of the pressures being measured. Further, an actual pressure wave for a heartbeat is much more complex, containing a great deal of information above that contained in simple systolic and diastolic measurements. It is only known that the pressure wave varies depending upon the location of the pressure transducer, in the case of an indwelling transducer-for example, a transducer that is located in a blood vessel will yield a different pressure wave than one located in the heart, and there are differences even within the heart.
Thus, some cardiologists use a much more complex and costly system than the simple pressure cuff, and it includes a pressure transducer attached to a catheter located in a blood vessel or the heart for generating an electrical signal representative of instantaneous pressure. This signal is coupled through the catheter and displayed on an oscilloscope or equivalent device where it may be studied directly or recorded, as by photographing. In this manner, the complex signal representative of the heartbeat can be studied or recorded in various locations throughout the circulatory system.
In many cases, a physician might want to have more information regarding blood pressure than it is possible to obtain by means of a simple pressure cuff, but, for his purposes, he may not need all of the complex information contained in an oscillographic trace or photograph. It may also be that he does not have available to him the expensive equipment necessary to make an oscillographic recording, or the skilled technicians needed to operate it. The present invention is thus designed to fulfill this need. That is, it provides the accuracy of invasive techniques and permits measurement in different parts of the circulatory system, but it is neither as complicated to operate nor as costly as an oscilloscope system.
The present invention has still another use, which will be described below, after the invention has been described in detail, and this relates to the use of the invention as a monitor for the insertion of a temperature transducer in the pulmonary artery. Such a transducer, located in the pulmonary artery, may be used to measure blood flow, as described in the co-owned application of Tuncay Emil, Ser. No. 594,210, filed July 9, 1975, for "SYSTEM FOR COMPUTING CARDIAC FLOW RATES FROM THERMODILUTION MEASUREMENTS."
Briefly, the present invention is a compact, portable self-contained unit which includes a resistive strain gauge transducer for sensing blood pressure from an indwelling catheter to generate a signal representative of instantaneous pressure. This pressure signal is coupled to the signal (i.e., negative) inputs of a bank of comparator circuits. The output terminals of the comparators are connected to associated ones of a string of light-emitting diodes (LEDs). The LEDs are connected in series across a voltage source comprising conventional 1.5 volt battery.
The other (positive) inputs of the comparator circuits are connected to associated nodes of a progressive reference voltage divider network. Thus, each comparator compares the instantaneous pressure signal with a reference voltage so that at any given time, all comparators having their signal (i.e., negative) inputs at a higher potential than the reference voltages on their positive inputs will have their outputs "LOW," and those comparators having their signal inputs at a lower potential than their associated reference voltages will have their outputs "HIGH." Thus, a single "LED" is energized at any given time, and it represents the instantaneous pressure value being measured.
The LEDs are mounted to the fact of a cabinet housing the circuitry in a vertical line, and a scale is provided alongside the LEDs for making the measurement. Preferably, the hash marks of the scale are in register with the LEDs to facilitate reading of the scale.
In a preferred embodiment, more than one scale is provided, for example, two scales may be provided--one on either side of the string of LEDs. A slide has a central opening with a width just sufficient to permit viewing of the scale and one reading range in a first position; and when the slide is adjusted laterally to a second reading position, the scale and a second range of readings may be viewed. The circuitry includes a switch responsive to the position of the slide so that when the range is increased, the gain of the system may be correspondingly decreased.
Circuitry is also provided for measuring the mean or average pressure; and this circuitry is activated by a push button switch, thereby permitting the operator to obtain updated mean pressure information.
A pair of cursors are carried by a vertical rod mounted to the slide showing the scale, and the cursors are slidable along the rod for marking reference pressures, if desired. This is particularly useful when the system is used during catheter insertion or while the catheter is in situ to indicate change in a previously measured value.
An optically isolated output is also provided for recording instrumentation, if desired.
The cabinet which houses the electronic circuitry and on which the scale is formed is provided with a base or stand permitting it to be supported on a horizontal surface, such as a cabinet or table top. At the same time, the base is provided with an I.V. pole clamp to permit continuous monitoring of a patient during transport.
In using the present system, one sees a moving dot or point of light, and although the dot moves in discrete increments, there is nevertheless some continuity because of the rate at which it is traveling. On the other hand, because the dots are associated side-by-side with the hash marks of a scale, it is easy to observe specific maximum and minimum values by fixing one's eye on the scale. In the low range scale, one can read to within 1 mmHg, and in the high range, pressures are measured in increments of 2 mmHg.
Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description of a preferred embodiment accompanied by the attached drawing wherein identical reference numerals will refer to like parts in the various views.